Laser surgery on the cornea of the human eye is an established method of treating visual defects caused by deviations in the shape of the cornea of the eye from the ideal shape. This involves removal, e.g. by ablation, of material from the cornea using a treatment laser, such as an excimer laser, for example.
In order to carry out the treatment, use can be made, in particular, of so-called spot scanning systems, wherein a treatment laser beam of the treatment system or the corresponding treatment laser beam spot, respectively, is moved over the cornea by means of a deflecting unit, also referred to as a scanning unit, according to a predetermined ablation program, and causes an ablation at predetermined locations. The lasers of use are characterized primarily by a small effective spot diameter, which allows to ablate small areas on the cornea and, thus, to locally modify the refractive power of the cornea and of the eye.
This treatment method allows patient-specific corrections, also referred to as “customized ablation”. These corrections comprise not only the sphero-cylindrical correction of visual defects, but also the correction of irregularities of the cornea, in particular also of spatially very small artifacts, and of higher aberrations, i.e. in particular also higher-order imaging errors characterized by high spatial frequencies, of the eye.
For treatment, first of all, the corneal topography is measured or the eye is examined, respectively, by means of aberrometers in order to detect irregularities or aberrations, respectively.
For the respective correction, ablation programs are then calculated, prior to surgery, by means of suitable programs, said ablation programs being based, inter alia, on empirical values for the ablation behavior of the cornea and defining the guidance and intensity of the treatment laser beam as a function of time. Using the treatment laser beam, which is emitted and guided according to the calculated ablation program, material is then ablated from the cornea.
The depths of removal in the cornea during correction of higher-order aberrations are usually only a fraction of the depths of removal required for correction of low-order refraction, in particular for sphero-cylindrical correction. Whereas sphero-cyiindricai corrections require a removal of about 12 μm per diopter on a 6 mm treatment pupil, in the correction of higher aberrations, the required local changes in power of refraction are achieved already by minor removal, i.e. generally already with one or few laser pulses.
Therefore, in sphero-cylindrical corrections, variations of system parameters of the treatment system, e.g. of the treatment laser parameters (such as, for example, the fluence of the laser), may average out statistically during ablation, so that a particularly exact adherence to the values of the system parameters is certainly desirable, but not critical. When correcting higher aberrations, however, a statistic compensation of variations in system parameters of the treatment system during ablation and, thus, smoothing of the ablation pattern on the cornea, may usually no longer be expected due to the low number of treatment laser pulses. The strict adherence to predetermined values of the system parameters is, therefore, critical for the correction of minimal details on the cornea and/or the treatment of visual defects which correspond to higher aberrations and are characterized by high spatial frequencies.
Therefore, on the apparatus-side, in addition to the standardization of the treatment atmosphere and the precision and speed of a system for tracking eye movements during treatment, which system may be, for example, a so-called “eye tracker”, specifically also the stability and quality of calibration of the laser system and of the deflecting device are important for the quality of the treatment.
In order to adhere to the predetermined values of the system parameters, the treatment systems are suitably adjusted both at the factory and later, during maintenance. In essence, two methods are known for this purpose.
In the so-called fluence test, a predetermined test film is treated by the treatment system, which comprises a treatment laser, according to an ablation program specifically designed for said test, whereby a corresponding pattern forms on the film. Local breakthrough thresholds in the test film allow re-adjustment of the pulse energy of the treatment laser. On the one hand, this method allows to approximately determine the half width of the spot diameter of the treatment laser beam. On the other hand, control information on the quality of the scanner system of the treatment system may be obtained.
In another method, given sphero-cylindrical PMMA lenses can be ablated using an ablation program for sphero-cylindrical correction. The refractive power of the lenses thus obtained can be determined, e.g. by determining the focal length by means of a so-called lensometer, for example, with a precision of ca. 0.1 diopters and can be compared with an expected desired value for refractive power. For calibration, the pulse energy is then re-adjusted.
In known treatment methods, the pulse energy or fluence of the treatment laser is measured on-line during treatment, allowing a re-adjustment during the ablation procedure of said treatment in order to improve treatment successes.
However, said methods do not allow determination of further system parameters of the treatment system, which may have a considerable influence in view of the above-mentioned precision requirements, in particular those of a “customized ablation”. Further, only indirect control of the ablation performance of the treatment system is obtained from the measurement of energy. The real progress of ablation during treatment cannot be measured, so that one has to rely on empirical values for re-adjustment. Therefore, fluctuations of the system parameters during treatment still limit the precision of treatment.